Phase or frequency modulated transmitter



Mayv30, 1939. G. ussELMAN vPHASE 0R FREQUENCY MODULATED TRANSMITTER 2 sheets-shaun 1 Filed Feb. 23, 1937 FREQUENCYMl/lT/PLIER SIG/VAL SOURCE AMPllF/fk I IL i .INVENTOR @.L USSELMN ATTORNEY NNW.

.SVGA/AL SOURCE May 30, 1939. G. L. UssELMAN IHASE OR FREQUENCY MODULATED TRANSMITTER I Filed Feb. 23, 1937 2 Sheets-Sheet 2 ATTORNEY Patented May 30, 1939 PHASE OR FREQUENCY MODULATED TRANSMITTER George Lindley Usselman, Port Jefferson, N. Y., assignorfto Radio Corporation of America, a corporation of Delaware Application February 23, 1937, Serial No. 126,954

9 Claims.

This invention discloses a method of and means for phase or frequency modulating high frequency wave energy. This is accomplished by producing a reactive effect and varying the said reactive effect in accordance with a signal. In a practical embodiment the reactance may constitute the anode tuned circuit of one stage of a transmitter.

In this system substantially no amplitude modulation is generated during the process of phase or frequency modulation. y

I n describing my invention reference will be made to the attached drawings wherein Figures 1 and 4 are basic circuit diagrams illustrating the manner in which the reactive effects are obtained in a single stage and in a push-pull stage respectively, and varied at signal frequency;

Figures 2 and 3 show two single tube stage modications of my novel phase or frequency modulation system; while Figure 5 illustrates one push-pull embodiment of my signalling system.

My invention as illustrated in Figure 2 comprises an exciter E, phase modulation elements or means A-B-C-F, audio modulation frequency amplifier or modulator elements D-Di-I-I-Ti and/or frequency multiplier I and antenna J.

More in detail the source of wave energy of the desired frequency at E'supplies wave energy by way of coupling and blocking condenser I to the grid 4 of tube F. Operating bias for grid 4 is supplied from source 6 by way of leak resistance 2. F may be of the screen grid type as shown and may have a screen grid 8 connected to a point on source l0. The anode l2 of F is connected to a tuned circuit or reactive network comprising inductance C, tuning capacities A-B in series in shunt to C and a choke inductance L shunting B. Choke L is provided to supply plate current to tube D. 'I'he wave energy transferred to F and from F to the network A-B-C as modified therein is impressed by way ofv coupling and blocking condenser i4 to a unit I which may include as desired, amplifiers, frequency multipliers, and amplitude limiters. The output from the unit I is impressed on an antenna system J. The antenna system J may of course be replaced by transmission lines or any other load circuit. A point on the inductance C is connected to the anode I8 of a modulator tube D', while a point between condensers A and B is connected to the anode of a modulator tube D. The control grids 22, 24 of tubes D1 and D respectively, are connected to the secondary winding 28 of a transformer T1, the primary of which is connected with any source of controlling potentials H. The

controlling potentials at l-I may be of tone frequency or may be characteristic of voice signals. Bypass or blocking condensers are shown marked as X. The purposes of other parts of the transmitter are apparent to those skilled in the art.

The operation of this invention is as follows, and is based on two fundamental facts. Given a parallel circuit consisting of a capacity and an inductance tuned to resonance at a given frequency, it becomes effectively a pure resistance and if a variable resistance or impedance is connected or tapped across a part of either leg or branch of the parallel circuit the tuned circuit becomes a variable impedance containing a variable reactive component. The second fact is that alternating current in passing through an impedance element containing reactance will have its phase displaced. In the present disclosure A-B-C of Figures 1 and 2 and A-B-C and A-BC' of Figures 4 and 5 are the tuned circuit described above while the shunting resistances or impedances or one of the same of all figures are the variable resistances or impedances described above.

In Figure 1 we have a parallel tuned circuit.

consisting of the inductive branch C and the capacitive branch A-B. Across a portion of the inductive branch we have connected in shunt variable resistor D. This resistance D represents the resistance of tube D of Figure 2. Also across:-

a portion of the capacitive branch A-B we have connected in shunt variable resistor D. This resistor D represents the resistance of tube D of Figure 2.

The operation of the invention may be moreV clearly understood by first referring to Figure 1 for explaining some of the principles. To begin, assume capacitance A-B and inductance C to be tuned to resonance at some desired frequency so that the impedance between points P1 and Pz is substantially pure resistance. Now, if we tap variable resistor D across a part of inductance C, the circuit will tune at a higher frequency and the impedance between points P1 and P2 will have an inductive reactance component at frequency (f). If we remove resistor D and instead tap variable resistor D across capacitor B which is a portion of the total capacitive branch of the circuit, the circuit will tune at a lower frequency and the impedance between points P1 and P2 will have a capacitive reactance component at frequency (f).

Now, returning to the drawings, if we connect both resistors D and D as mentioned above and as shown in Figure 1 assuming a balanced condiinductive reactance and vice versa dependingV upon the values of resistors D and D.

Applying the above principles -to a circuit as shown in Figure 2 modulator tubes D' `and D fulfill the purpose of resistors D and D in Figure 1. 'Ihe anode current in these tubes'constitutes the resistance which is varied by varying -the amount of anode current. This is done differentiallythrough transformerT from audio signal source H as shown in Figure 2. It will be noted that anode current for tube D' is drawn from tank coil C and that anode current for tube D is supplied through-choke L which is connected to one end of coil C as shown.

As has been explained above, a change in the reactance of the tuned anode circuit such as A--B-C, changes the phase of the electrical oscillations in it as compared to the grid excitation oscillation. Therefore, if tubes D and D are modulated differentially .according to the signal the phase of the energy in the tank Vcircuit of transmitter tube F will be phase modulated according to the signal. This phase modulated energy will also be substantially free from amplitude modulation for as explained in connection with Figure '1, the resistive component between P1 and P2 remains substantially constant.

This phase modulated energy may be repeated, amplified, limited in amplitude,.and/ or multiplied in frequency inother stages such as I in Figure 2 before it is radiated by antenna J.

Other modifications of this invention could be suggested such as the one shown .in yFigure 3. In this case the modulated stage is an oscillator, having a tunedgrid circuit 3|!coupled by a condenser N to the parallel circuit A-B-C connected to the anode. The grid circuit 3l! is shown as a standard tuned circuit. oscillations are produced by virtue of interelectrode capacity and/or feedback through N. The remaining portions of this oscillator which is phase or frequency modulated will be understood when considered in the light of the foregoing description of `the previous-figures. If a crystal be substituted for grid circuit 30 the transmitter will deliver a phase modulated signal. 'I'he same could be said if grid circuit 30 is a resonant line. However, if a high power factor or highploss grid circuit such as shown at 30 in Figure 3 is used the transmitter will deliver mostly a frequency modulated carrier signal. This because there is `little stored energy in circuit 30 and therefore the frequency of excitation energy in circuit 30 will follow that of the tank circuit A, B and C.Y In other words, the stability of `the grid circuit 3U is low and will follow the frequency of the feedback energy to produce in the oscillator frequency modulated waves. The phase and frequency modulated energy from the oscillator may be repeated,

`amplified and/or' multiplied in frequency in one -or more stages -such as Iloeforev being .radiated by antenna J. The transmitters shown in Figure Vbetween B and B to a point between-C and C C. should be kept constant except the adjustment l0 necessary for tuning. In other words, if capacitor B is decreased by a certain amount then capacitor A should be increased by the same amount and vice versa to maintain the tuned condition intank circuit A, B and C. l5

While I have illustrated in the preceding figures the application of my novel phase modulating sChcmetosingle ended or single tube stages it will kloerealized that the same is also applicable to double ended or push-pull stages. When the invention is applied Ato double ended or push-pull stages the fundamental circuit illustrating the manner in which the reactive effects are produced and-.changed at signal frequency will be as illustrated in Figure 4. In this fundamental diagram as in Figure 1, A and B represent capacitive reactances. D represents a variable resistor in shunt to B while D represents an additional variable resistor connected in shunt to a portion of C. Here, since a push-pull effect is involved. additional inductance C' and additional condensers A B are used. Thus, we have condensers A B B A in series in shunt to C and C' withY a choking inductance L2 connecting a point and to ground. The variable resistances D and D of Figure 1 are supplemented by additional similar Variable resistances 2D and 2D.

While it is believed the push-pull modification as illustrated in Figures 4 and 5 ywill be readilyq-,go understood by those skilled in the art, the operation thereof will be set forth here, even though it is ,in part aY repetition of the statement `vof operation of the modifications shown in Figures `1, 2, and 3.

To more or less repeat my previous disclosure, assumecapacitance A, B, A and B', and inductance E to be tuned to resonance so that the vimpedance between pointsP1 and P2 is substantially pure resistance for alternating currents..,50 If we tap variable resistors D and 2D across a part of inductance C as shown, the 'circuit will tune at a higher frequency and the impedance Vbetween points P1 and P2 will have an inductive reactive component. If we remove resistors D'ali and 2D and instead `tap variable vresistors D and '2D across capacitors B and B which is a .portion of the total capacitive branch of the circuit, as shown the circuit will tune at a lower frequency and the impedance between pointscO P1 and P2 Will have a capacitive reactance component.

Now, if we connect variable resistorsD and 2D and D and 2D' as mentioned above and as shown in Figure 4, assuming a balanced condition onf`45 each side of the tuned circuit, right and left and also above and below the grounded central point,

-the impedance between points P1 and P2 will again be substantially pure resistance.

effective resistance component between points P1 'and Pz will remain substantially constant but the reactive component will swing or oscillate from capacitive' to inductive reactance and vice-versa .depending upon the values of resistors D-2D and D-2D'.

Inthe modification shown in Figure 5 as in the prior figures, E represents any source of wave energy to be modulated. This source of wave energy is connected by coupling condensers I to the control grids of a pair of tubes F, F', the

anodes of which are connected to the reactive net- Here, as in the prior modification the reactance of the network is controlled by modulator tubes D and D connected in phase opposition between a source of modulating potentials H and the said network. The tubes D and D are in modification of any appropriate type having two anodes. For example, they may be tubes of the type RCA-19 and RCA-79 or RCA-53. In operation the grids may be tied together to operate in phase as a single grid. The anode to cathode impedances of D represent resistors D and 2D and are varied in phase. 'Ihe anode to cathode resistances of D represent resistors D and 2D and are varied in phase.

Applying the above principles to a push-pull circuit as shown in Figure 5, modulator tube D with anodes 40 and 42 fulll the purpose of resistors D and 2D, while modulator tube D with anodes 44 and 46 fulfill the purpose of resistors D and 2D. D' constitute the resistances which are varied by varying the amount of anode current. This is done differentially through transformer T from signal ysource A acting on the grids of tubes D and D as shown in Figure 5. It will be noted that the anodes 44 and 46 of tube D draw current from tank coil CC and that the anodes 40 and 42 of tube D are supplied with current from choke coil L which is connected to the center of tank coil CC as shown through choke coil L2. It will also be noted that the anodes 40 and 42 of tube D are connected across equal impedances on each side of the neutral point of the tank circuit condensers and that the anodes 44 and 46 of tube D are connected also across equal impedances each side of the neutral or central point of the tank circuit coil as shown in Figure 5. This is to insure equal or balanced modulator action each side of the push-pull circuit for the prevention of amplitude modulations.

It has been explained hereinbefore that a change in the reactance of the tuned anode circuit, suoh as A, B, C, A', B', C' changes the phase of the electrical oscillations in it as compared with the grid excitation oscillations. Therefore, if tubes D and D are modulated differentially according to the signal the phase of the carrier energy in the tank circuit of the transmitter stage will be phase modulated according to the signal. This phase modulated energy should be substantially free from amplitude and other spurious modulation for reasons explained in connection with Figure l.

The phase modulated energy may be repeated, amplified and/or multiplied in frequency in other stages such as I in Figure 2 before it is radiated by antenna D.

Other modications of Figure 5 can be suggested in which tubes F and F1 may be of the screen grid type. Tubes D and D' could be replaced by four single anode tubes.

Another modification could be made by omit- The anode current in tubes D and ting exciter E of Figure 5 and introducing feedback energy into grid circuit G from the anode circuit in any suitable manner. This would make this stage B an oscillator. If grid circuit G has low power factor such as a resonant line or a quartz crystal changes in anode tank circuit reactance of stage B will produce phase modulation. If grid circuit G has high power factor or if this grid circuit is not resonant then changes in the anode tank circuit reactance of stage B will only of the reactance in onev of said branches, a:

second variable impedance shunting a part only 0f the reactance in the other of said branches, means for impressing wave energy on said circuit, means for deriving wave energy from said circuit and means for varying said impedances in accordance with controlling potentials.

2. In a system for modulating the phase of wave energy in accordance with signals, a circuit tuned to resonance at the mean lfrequency of said wave energy, said circuit comprising an inductive branch and a capacitive branch, said capacitive branch comprising capacities in series, a Variable impedance shunting a portion of said capacitive branch, a variable impedance shunting a portion of said inductive branch, means for impressing wave energy on said circuit, means for deriving wave energy from said circuit and means for varying said impedances in opposite senses at signal frequency.

3. In a phase modulation system, an electron discharge device having an anode, a cathode, and a control grid, alternating current circuits connected between said control grid and cathode and between said anode and cathode, means for causing wave energy of the desired frequency to flow in said circuits, one of said circuits comprising parallel reactances and being resonant at the frequency of said wave energy, a variable impedance in shunt to a part only of one of said re. actances, a variable impedance in shunt to a part only of the other of said reactances, and means for varying said impedances in displaced phase relation at signal frequency.

4. In a phase modulation system, an electron discharge device having an anode, a cathode, and a control grid, alternating current circuits connected between said control grid and cathode and between said anode and cathode, means for causing wave energy of the desired frequency to flow in said circuits, one of said circuits comprising parallel inductive and capacitive branches and being resonant at the frequency of said wave energy, a variable impedance in shunt to a part only of one of said branches, a variable impedance in shunt to a part only of the other of said branches, and means for varying said impedance in displaced phase relation at signal frequency.

5. In a phase modulation system, a plurality of electron discharge devices each having an anode, a cathode, and a control grid, alternating current circuits inter-connecting said control grids and cathodes and said anodes and cathodes, means for causing wave energy of the desired frequency to flow in said alternating current circuits, one of said alternating current circuits including capacitive -and -inductive reactances in riparallel V:and rbeingcparallel resonant Vat the frequency :of said wave-energy, a -variable impedance connected in'shunt to a part-only of one of :said :,reactances, a second variable ,impedance connected Ain shunt toa part-only of the other voi? :said resistances, andmeans for varying said impedances atslgnal-frequency.

6. Inra rphase modulation system, aV plurality of .'electron discharge devices each having an anode, a cathode,.and-acontrolrgrii alternating current circuits inter-connecting saidcontrol gridsand' cathodes and saidv anodesand cathodes, -means for causing -wave energy lof the desired frequency yto ilow in said alternating current circuits, one of said alternating current circuits including a capacitive branch and an inductive branch, a'fvariable resistor connected'in shunt to each rof said 'branches, and means for varying saidresistances in phase 'opposition at signal frequency.

7. In a phase modulation system, a plurality of `electron discharge devices each having an anode,

a cathode, and a control grid, alternating currentcircuits inter-connecting said control grids and cathodes, other alternating current circuits inter-connecting said anodes and cathodes, means-for causing v,Wave energy'of the desired Afrequency to flow in said alternating current circuits,=said-otherof said alternating current circuits including fa capacitive branch and an inductive branch tuned to the mean frequency of said wave energy, a variable resistor connected 1in-shunt tofeach oisaidbranches,l and meansfor `varying -said fresistances in phase oppositionwat signal frequency.

-8. The method of modulating the wavelength Vof Wave energy by means of a circuit comprising f inductive and capacitive branches in 'parallel tuned to resonancegat the vmean frequency of vsaid YWave-energy lwhich includes `the steps of,

impressing said wave energy Von said circuit, de-

rivingfrwave energy `from said circuit, impressingwr a resistive'component onveach leg of'said circuit,

.andyaryingsaid resistive components in phase opposition=at signal frequency, to therebyvvary thereactive components of said 'circuit Vat signal frequency. 9. In a wave lengtlrmodulationsystem, an velectron discharge device having an anode, a` cathode, and a control Vgridyalternating `current circuits connected between'said control grid Vand cathode ,and`between saidanode and cathode, means for@ causing Wave energy of the desired frequency to flow in said circuits, the alternating current cir- 

